Unilocular echinococcosis or hydatidosis is a parasitic disease due to the development in man and also livestock of the larval form of a small cestode, Echinococcus granulosus. The dog represents the reservoir of the adult parasite. This parasitosis is a genuine scourge in breeding countries in Africa, in Latin America and in Australia, as well as the countries bordering the Mediterranean basin, where there are economic repercussions in both the medical and the veterinary field. While almost no diagnostic test is at present available for animals, medical diagnosis is often intricate on account of the non-specificity of the symptoms observed. On the other hand, several standard immunological methods, such as haemagglutination and the complement-fixation reaction, have been performed for the detection of hydatidosis. Hydatid fluid, used as a source of antigens, contains, in addition to the products of metabolism of the parasite common to other helminths, several components of the host (CAPRON A. et al. (1), RUSSI S. et al. (2), BEN-ISMAEL R. et al. (3). This gives rise to the problem of false-positive reactions. However, the almost constant presence of antibodies that precipitate an antigen designated antigen 5 has been shown by the immunoelectrophoresis technique (CAPRON A. et al. (4) and (5)). Since then, this test has remained one of the best immunological reference tests for any epidemiological study of hydatidosis. Antigen 5, which is highly immunogenic, has formed the subject of several studies showing that it is a thermolabile lipoprotein capable of binding to concanavalin A, of relative molecular mass approximately 60 kDa (ORIOL R. et al. (6), BOUT D. et al. (7), PIANTELLI M. (8)). Analysed under reducing conditions, this antigen dissociates into two subunits of 37 and 22 Dka. Using more sensitive techniques such as ELISA, the diagnostic potential of purified antigen 5 has been amply confirmed. Anti-antigen 5 antibodies have also been detected in sera of patients or animals infected with E. multilocularis or with other cestodes such as Taenia hydatigena, T. ovis and T. solium. However, their level remains low. These few observed false-positive reactions have been attributed in part to the presence of a phosphorylcholine epitope on antigen 5.
The current problem is to be able to have at one's disposal specific parasitic antigens which are well standardised and in sufficient quantity for the diagnostic tests. This is the standpoint from which the studies currently being carried out on hydatidosis, and which are based on monoclonal antibody and genetic engineering techniques, are endeavouring to isolate and identify parasitic components possessing a high degree of specificity.
The objective of the present invention is to provide a parasitic antigen specific to hydatidosis.